Applicant's U.S. Pat. No. 4,636,907, issued Jan. 13, 1987, entitled "Arcless Circuit Interrupter" in the name of E. K. Howell and assigned to the assignee of the subject application, is hereby incorporated by reference. The above referenced U.S. Patent relates to modifying, i.e. interrupting, load current flow in a first circuit that interconnects a source of electrical energy and a load circuit. Load current flowing through the first circuit is temporarily diverted to a second, i.e. diversion, circuit. Upon load current diversion, a switch in the first circuit can be rapidly opened under substantially zero current conditions and thus without arcing. Diversion of the load current prior to switch opening is accomplished by a controlled impedance circuit in the first circuit. The switch and the controlled impedance circuit are serially connected between the source of electrical energy and the load circuit, and the diversion circuit is connected in parallel with the series combination of the switch and the controlled impedance circuit. Various types of diversion circuits may be utilized for this purpose. Representative diversion circuits are, for example, disclosed in the following U.S. Patents which are in the name of E. K. Howell, the subject applicant, are assigned to the assignee of the subject application, and are hereby incorporated by reference: U.S. Pat. No. 4,700,256, issued Oct. 13, 1987 (which is a Continuation-In-Part of U.S. patent application Ser. No. 610,947 filed May 16, 1984 and since abandoned) entitled "Solid State Current Limiting Circuit Interrupter" and U.S. Pat. No. 4,631,621, issued Dec. 23, 1986, entitled Gate Turn Off Control Circuit".
Load current diversion to the diversion circuit is produced by the controlled impedance circuit. During normal operation, i.e., prior to diversion, the controlled impedance circuit essentially has a very low voltage drop and thus low power dissipation. Load current diversion results from a control signal which effectively increases the voltage drop across the controlled impedance circuit. This voltage causes the transfer of the load current and of energy stored in the inductive components of the first circuit to the diversion circuit. This is, for example, further described in U.S. Pat. No. 4,723,187, issued Feb. 2, 1988, entitled "Current Commutation Circuit", which is also in the name of E. K. Howell, is assigned to the assignee of the subject application, and is also incorporated herein by reference.
Controlled impedance circuits used for load current diversion must meet various requirements. When switched to their high impedance state, load current flow must produce a voltage drop that is sufficient to transfer current and stored energy at a sufficiently high rate.
While operating in their normal low impedance state, i.e. prior to diversion, load current must flow through the controlled impedance circuit with minimal power dissipation. U.S. Pat. No. 4,636,907, issued Jan. 13, 1987, discloses, for example, controlled impedance circuits comprising a switchable solid state device whose main electrodes are connected in circuit with the switch, source of electric energy and the load circuit. During normal operation, the solid state device is turned on so as to operate in saturation. When diversion is commanded, a control signal switches the solid state device to a high impedance, i.e. OFF state, so as to produce a voltage drop across the main electrodes. Particularly with large load currents, it is vital that the switch exhibits in its ON state an extremely low voltage drop and thus extremely low power dissipation. However, many types of solid state devices, e.g. certain types of thyristor structures and bipolar transistors, exhibit significant junction voltage drops in their ON state. With large load currents, this can produce substantial power dissipation.
A further requirement applies to arrangements for diverting a-c, as opposed to d-c, load currents. When a-c load currents are to be diverted, the controlled impedance circuit must be capable of being switched to its OFF state during either half-cycle, i.e. polarity, of the load current and source potential. If the controlled impedance circuit comprises a switchable solid state device whose main electrodes are connected in circuit between source and load, the solid state device must be capable of bilateral operation. Specifically, it must be capable of being switched OFF despite polarity reversals across its main electrodes. However, many types of solid state switches, e.g. certain thyristors, bipolar transistors and field effect devices, do not exhibit this type of bilateral operation.
U.S. Pat. No. 4,636,907, issued Jan. 13, 1987, also discloses an alternative embodiment for a-c load current diversion and interruption that satisfies the above recited requirements. This couples a-c load current via a transformer and bridge rectifier across the primary electrodes of bipolar transistors connected as a Darlington pair. The transformer has a primary in series with the switch of the load circuit and a secondary step up winding connected to the input of the bridge rectifier. When the bipolar transistors are gated to saturation conduction, the primary winding has an extremely low voltage drop. When the bipolar transistors are gated off, the voltage across the primary winding increases sufficiently to divert load current to the diversion circuit. The bridge rectifier provides a unilateral potential across the primary electrodes of the bipolar transistors. It thus compensates for any inability of the transistors to switch satisfactorily when a-c potential is directly applied across their primary electrodes. An adequate turns ratio of the transformer also assures that the primary winding has a sufficiently low voltage drop and power dissipation during normal operation, but a sufficiently high voltage drop for load current diversion in response to an interruption command. As subsequently described, the circuit including the bridge rectifier and bipolar transistors can have a substantial minimum voltage drop during saturation conduction. For these reasons, an adequate transformer step up ratio is required to maintain a sufficiently low voltage drop across the primary. Careful design is therefore required to also provide a sufficient voltage drop across the primary winding, when the transistors are cut off, to assure that load current is diverted. The relatively high voltage across the transformer secondary also requires use of solid state devices having a sufficiently high blocking voltage. Devices with a high blocking voltage may have relatively high voltage drops during saturation so as to require even more careful circuit design. Also, the use of power devices having a high blocking voltage, as well as the transformer, result in increased production costs.